The present invention pertains in general to methods for multiple zone, compressible fluid injection and in particular to methods for compressible fluid injection into multiple zones of a hydrocarbon bearing formation using a single tubing string.
An oil-producing well may pass through several petroleum containing strata or sand members, i.e. producing zones separated by non-producing zones. These producing strata may differ in permeability, homogeneity and thickness. Furthermore, the petroleum in these producing strata may differ in amount, viscosity, specific gravity and average molecular weight.
Compressible fluids are commonly injected into oil formations to enhance oil production. Compressible fluids are defined as fluids that can achieve sonic flow when passed through a restriction. For a given set of upstream conditions, the flow rate of a compressible fluid through a restriction will increase as the absolute pressure ratio (P.sub.downstream / P.sub.upstream) decreases until the linear velocity of the compressible fluid in the restriction reaches the local speed of sound. When sonic flow is reached, the flow no longer responds to changes in the downstream pressure.
Examples of compressible fluids are gases such as air, N.sub.2, CO, CO.sub.2, CH.sub.4, flue gas, natural gas, dry steam and the like, and mixtures of two phase fluids like gases and liquids such as wet steam. Wet steam is defined as steam that has a liquid phase, i.e. less than 100% vapor phase steam. For example, 80% quality steam has a liquid phase of 20% by mass.
Where petroleum within a stratum is so viscous that the temperature and pressure within the stratum are insufficient to cause it to flow to a producing well, hot fluids, particularly steam, are injected into such strata in order to raise the temperature of the stratum and thereby reduce the viscosity of the petroleum contained therein to a point at which the petroleum be moved to a producing wellbore. Oil production may also be enhanced by injection of gases such as nitrogen, carbon dioxide or flue gas alone or in combination with steam.
In wells containing multiple producing zones, it may be desirable to simultaneously treat more than one stratum with compressible fluids at the same time. These strata may require different rates of injection to optimize production therefrom. For typical compressible fluid injection, injection tubing is run into wells within the casing to each production zone. Packers are placed between the tubing and the casing above and sometimes below the stratum to be injected. Next, the wellhead is connected to a source of compressible fluid, such as a steam generator, and the fluid is pumped into the stratum formation through the tubing. The steam quality is either not monitored or only controlled at the surface as taught by U.S. Pat. No. 4,149,403. Thus the exact quality of the steam and the actual injection rate down the wellbore at the producing zone is not accurately known. In addition, very deep formations would require an excessive wellhead pressure because of the pressure loss across the surface choke and the frictional losses as the compressible fluid moves down the tubing.
Another method of injecting fluids simultaneously into different strata involves employing multiple channels with each channel injecting fluid into different strata. For example, the concentric tubing strings of the sort shown in U.S. Pat. No. 4,399,865, are formed by running a first steam-bearing pipe within a second to form two flow channels. The concentric tubing acts as a long heat exchanger which tends to plug up when used with hard water steam injection. Still another method utilizes a multichannel conduit of the sort shown in U.S. Pat. No. 4,424,859. The conduit is composed of a plurality of contiguous flow channels within a cylindrical shell. The cost of the injection operation and the efficiency would be improved if, preferably, a single tubing string could be utilized.
U.S. Pat. No. 4,248,302 teaches the use of a dual tubing strings with side pocket mandrels which incorporate "constant flow regulators or orifice regulators". However dual tubing strings will not fit into small diameter casings found in many wells. In addition, the reference does not teach "constant flow or orifice regulators" which operate or function on the basis of sonic flow conditions. The "Model `BF`" downhole flow regulator specified by the reference was designed for water, a non-compressible fluid. It operates by varying a port opening in response to a change in either tubing or formation pressure, i.e. it throttles the flow of fluid which is not at sonic flow condition. In addition, it is generally desirable for downhole tools to be without moving parts for simplicity and reliability.
In small diameter casings which have room for only a single tubing string, it may be desirable to inject fluid into more than one strata from that single tubing string. Typically, an injection tubing string with an open end is hung inside a casing which is perforated at each producing zone. In another method, the casing is perforated and holes are drilled in the tubing at the producing zones. The tubing is packed off within the casing above and below the perforations. When injecting a compressible fluid such as steam, it is desirable to maintain at predetermined values both the quality and flow rate of steam injected into each producing zone. Heretofore, the split between producing zones of compressible fluid injected down a single tubing string could not be accurately controlled.
Injection rate depends on tubing fluid pressure, formation pressure, and the size of injection ports (for example the tubing holes). Since these pressures can change, (particularly the formation pressure will change during the period of injection life) injection rates into one or more producing zones are not readily controllable. Pressure and spinner surveys generally indicate that most of the steam tends to flow into the producing zones or adjacent non-producing zones that have the lowest pressure and highest permeability. Non-producing zones such as water bearing zones, tend to preferentially divert the vast majority of the steam away from the producing zones. These tendencies drastically increase costs and reduce production of hydrocarbons.
Thus, there is currently a need for a practical means and method to control the distribution of compressible fluids and particularly steam between different producing zones and at predetermined rates. Preferably, there is a need for this to be accomplished with a single tubing string injecting into more than one producing zone.